When chlorophyll molecules are light excited, their electronic excitation resides largely within the macrocyclic ring (chlorin); no preferential or charge transfer excitation takes place at the peripheral C C-double bond or isocyclic C O-double bond. On the other hand, phytochrome with its open form of the porphyrin macrocycle show a significant charge transfer and local excitation involving the photoreactive ring A. In the case of psoralen (skin carcinogen in the presence of near UV), the excitation energy accumulates strongly in the 3, 4-C C-double bond in the triplet state, thus accounting for its photoreactivity toward DNA.